This invention relates generally to a fusing apparatus for an electrophotographic machine, and more particularly to such an apparatus wherein a toner powder image formed on a support sheet is fused thereon by direct applications of heat and pressure with the use of a heat roll.
As is well known in the art of automatic copying machines using an electrophotographic process, an electrostatic latent image corresponding to an original to be copied is formed on a photosensitive member by exposing the original following the uniform application of electrostatic charges on the photosensitive member. The thus formed latent image is developed with a finely divided developing material or toner to form a powder image which is then transferred onto a support sheet such as paper. The support sheet bearing the toner powder image is subsequentially passed through a fusing apparatus and is thereafter discharged out of the machine as a final copy. There are two generally known types of fusing processes used in carrying out the fusing of the toner powder image. The first is an oven-type in which heat is applied over a wide portion of the support sheet, and the second is a heat-roll type in which the support sheet is simultaneously pressed and heated by a heat roll as it passes therethrough. Compared to the first type of fusing approach, the heat roll type is more efficient in that the time required for fusing the toner image is substantially reduced since the toner image is heated as it is directly compressed, and furthermore the size of the copying machine itself can be minimized because of the reduced space required for the heat roll type fusing apparatus.
However, because of the narrow proper temperature range required for the heat roll type fusing apparatus, the toner powder image will not be properly fused or fixed on the support sheet if the required surface temperature is not maintained. Thus, adhesion of the toner on the heat roll surface results thereby causing transfer of the adhered toner to the following support sheets at the fusing station. This phenomenon is generally referred to in the printing art as low temperature transfer or "offset". On the other hand, if the surface temperature of the heat roll is higher than the proper temperature required, the toner becomes over-fused and adheres on the heat roll surface simultaneously with the fusion on the support sheet so that adhered toner will be transferred to the next support sheet. This phenomenon is likewise referred to as "offset" or high temperature transfer. Another drawback with the use of the heat roll type fusing apparatus is that the support sheet, to which the toner image is fixed, often becomes wrapped about the heat roll thereby causing jamming due to the cohesive nature of the toner. In order to avoid the above described offset phenomenon, the heat roll is coated with a non-cohesive material such as silicone, rubber, silicone oil or is provided with a coating of Teflon, a DuPont Corporation product composed of tetrafluoroethylene resin. Although such coating prevents toner offset to a certain extent, a completely satisfactory fusing of toner powder images cannot be expected.
For fusing the toner powder image, it is necessary to apply adequate pressure between the heat roll and a support roll in contact therewith while the support sheet bearing a toner powder image passes therebetween during the fusing mode of the copying machine. However, since both rolls are coated with a non-cohesive material over their surfaces, if the pressure applied between rolls is made while the fusing apparatus is at rest, that is, while both rolls are not rotating, the roll surfaces are apt to become dented or uneven so that a non-uniform pressure is likely to be applied to the support sheet. To avoid such a problem, the fusing apparatus in accordance with one embodiment of the present invention is so designed that the rolls are pressed together with a sufficient pressure while the support sheet passes therebetween, and the rollers are separated when such support sheet is not disposed therebetween. Thus, no heating of the support roll occurs while the rolls are separated. An effect similar to a decrease in the heat-discharging area of the apparatus is therefore obtained with an accompanying improvement of the energy efficiency. In addition, the stand-by period for the fusing apparatus, that is the time required for the fusing apparatus to reach a proper fusing temperature, is reduced. Furthermore, the durability of both rolls is improved since there is no application of mechanical stresses thereon unless the support sheet is located therebetween, and since there is no continuous heat load under pressure on the support roll. Also, in the heat roll type fusing apparatus, heating of the toner powder image on the support sheet is effected by a single face application of heat from the heat roll only, and this is advantageous in that there will be no variance in the degree of fusion regardless of thickness or quality of paper used.
However, even if the heat and support rolls are separated at the time the support sheet is not located therebetween, both rolls must be continuously rotated while the machine is operating since, if the rolls are at rest, certain portions of both rolls will become over-heated to possibly exceed their heat resisting temperatures and thereby useless. This is particularly true in an arrangement wherein the heat roll is heated by an external heat source located immediately adjacent the roll. And, even if the heat roll is provided with a heat source located internally thereof, transfer of heat therefrom may cause a limited portion only of the support roll to be heated unless it is continuously rotated.
If both rolls are positively driven by means such as a gear arrangement, the difference in peripheral speeds on each roll will develop due to a difference in elasticity between both rolls. Such therefore causes the support sheet contacted by both rolls to possibly wrinkle, and to cause frictional wear of the roll surfaces. Furthermore, a quite complicated drive system will be required because of the need to intermittently separate the rolls. In addition, a sharp drop in surface temperature of the heat roll may take place during roll separation because of the transfer of heat to the cold support sheet at the moment of contact of both rolls. This may accordingly effect a low temperature transfer of toner powder image.